1. Field of the Invention
The present invention relates to an inner surface grinding tool which is suitable for precision machining, for example, a plurality of journal bearings of an engine, in a simultaneous fashion.
2. Background Art
As a technique for increasing diametric machining accuracy by expanding and contracting a diameter of a machining tool in grinding an inner surface of a workpiece, there is known a technique in which a diameter of a grinding tool is controlled by a cam member which is driven by an expansion threaded rod, for example, when cylinder bores of an engine are individually machined. (refer to JP-A-06-190713, for example).
In addition, in machining, journal bearing portions which bear a crankshaft of an engine, for example, there may be a case where a simultaneous multiple-position machining inner surface grinding tool is used which simultaneously machines a plurality of machining target portions.
In these related-art techniques, however, in a simultaneous multiple-position machining inner surface grinding tool which machines simultaneously a plurality of holes, since there is no technique for individually controlling diameters of grinding blade portions, in controlling the diameters thereof uniformly, for example, there has been caused a problem that diametric accuracy varies due to a rigidity of the workpiece, influence by a variation in initial cutting capabilities of the grinding blade portions or degree of propagation of wear in the grinding blade portions. In addition, in machining journal bearing portions with a simultaneous multiple-position machining tool which has no general expanding and contracting mechanism, in order to prevent a finished surface from being damaged from an interference of the simultaneous multiple-position machining inner surface grinding tool with the finished surface when the same tool is pulled out of a central hole in the workpiece, the simultaneous multiple-position machining inner surface grinding tool is designed to be inserted into and pulled out of the central hole in the work piece with an arbor center offset relative to a workpiece center. Due to this, only one tool can be mounted for each journal bearing portion, resulting in a problem that the machining efficiency is deteriorated.
Further, in simultaneously machining the plurality of journal bearing portions, since the total length of the machining target portions becomes long, the overall length of the arbor also becomes long inevitably. Moreover, in the related-art inner surface grinding tools, since it is difficult for the grinding blade portions to be laid out in opposing positions relative to the axis of the arbor, no balanced machining has been able to be implemented. Because of this, the arbor has to be subjected to a plurality of machining loads simultaneously from one direction and hence becomes easy to be deformed, leading to a problem that a required or designed accuracy becomes difficult to be obtained.